Since our experience with RTTs is relatively new, we cannot be sure that RTTs that are considered secure today will still be secure in the future. In particular, we must assume that once RTTs are used to secure something of value they will become the target of many hacking attempts, and attackers might find a way of efficiently breaking RTTs. We should therefore be prepared for the event that RTTs that are used by the system are broken. The analysis below also applies to the situation where an adversary is not able to technically break the RTT, but can obtain a large amount of human help to answer RTTs.
Identifying a successful attack : The first challenge is identifying that attackers are able to break the RTTs that are used by the system. The following techniques will help to identify large scale break-in attempts. During normal operation we can expect a certain fraction of login attempts to pass the RTT but fail in the password entry. These are mostly due to users that mistype their passwords, or to amateurs that run manual dictionary attacks. When the RTT is broken by an automated program, or possibly by attackers that organize “sweatshops” of low paid workers that are solving RTTs, this fraction should go up noticeably (once the RTT is broken the attackers can pass this test, but should still find it hard to guess the correct password). When the system recognizes that the fraction of unsuccessful login attempts that solve the RTT increases, it should assume that the RTT is broken.
Countermeasures : Once it is recognized that attackers can break the RTT, the first line of defense should be increasing the fraction p of login attempts for which an RTT is required. (At an extreme, the system can require more than one RTT to be solved for every login attempt, until it finds a different solution against the attack. This corresponds to a value p greater than 1, in terms of calculating the expected number of RTTs that have to be broken). This step requires the attacker to solve more RTTs for every account that it attempts to break. If the attack is conducted using human workers, or is computationally intensive, then the operation of the attacker
is slowed down until it recruits more human workers or installs more RTT breaking servers. We should therefore examine whether the fraction of login attempts that pass the RTT but fail the password entry decreases when p is increased. If this is not the case, then we should assume that the attacker has found a scalable way of breaking RTTs. The system must therefore change the RTT that is being used, in hope that the attacker does not have an efficient way of solving it. (This change does not slow down human attackers, but we believe, based on the quantitative analysis presented above, that the cost of human attacks is too high for them to be effective, e.g.
requires almost a week of human work per account.) The operators of the system must therefore have a set of different RTT tests. Once a successful automated attack is detected the system should switch to a new test, preferably taken from a new domain and sufficiently different from the existing test.
Additional security against attacks on individual accounts can be provided in the following way. When the server notices that there is a high number of failed login attempts to one (or a small number of) accounts, a flag is raised warning the security administrator that with high probability an attack is mounted on these accounts. The system could then increase, for these individual accounts, the fraction of login attempts that require RTTs, or even change the type of RTTs that should be solved, in order to slow down an attacker. The administrator may also contact the user via an out-of-band channel, such as email or phone, to agree on an alternative login method. The key advantages of the RTT scheme in this case are that (1) the security administrator is given significantly more time to react than in ”conventional” schemes where an account can be locked within seconds (e.g. after 5 unsuccessful login attempts), and that (2) a legitimate user will still be able to login to his account during this time period.